diff --git a/llvm/lib/MC/WinCOFFObjectWriter.cpp b/llvm/lib/MC/WinCOFFObjectWriter.cpp
--- a/llvm/lib/MC/WinCOFFObjectWriter.cpp
+++ b/llvm/lib/MC/WinCOFFObjectWriter.cpp
@@ -169,6 +169,7 @@
     Strings.clear();
     SectionMap.clear();
     SymbolMap.clear();
+    WeakDefaults.clear();
     MCObjectWriter::reset();
   }
 
diff --git a/llvm/test/MC/COFF/weak-uaf.ll b/llvm/test/MC/COFF/weak-uaf.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/COFF/weak-uaf.ll
@@ -0,0 +1,12 @@
+; RUN: llc --compile-twice -mtriple=x86_64-pc-win32 -filetype=obj < %s
+
+; UAF when re-using the MCObjectWriter. does not leak into the output,
+; but should be detectable with --compile-twice under ASAN or so.
+
+define weak void @foo() nounwind {
+  ret void
+}
+
+define weak void @bar() nounwind {
+  ret void
+}